Recently, various fuel cell modules in which a cell stack composed of a plurality of fuel cells capable of acquiring electric power using fuel gas (hydrogen-containing gas) and air (oxygen-containing gas) is contained in a housing, or various fuel cell devices in which the fuel cell modules are contained in an exterior case have been proposed as next-generation energy (for example, Patent Literature 1).
In such fuel cells, a fuel electrode layer containing Ni and ZrO2 solid solution having a rare-earth element, a solid electrolyte layer containing ZrO2 solid solution having a rare-earth element, and an air electrode layer formed of Sr-containing perovskite composite oxide are stacked in this order on a conductive support substrate.
However, since Sr contained in the air electrode layer diffuses into the solid electrolyte layer or Zr contained in the solid electrolyte layer diffuses into the air electrode layer in the course of manufacturing the fuel cells or generating electric power, there is a problem in that a high-resistance reaction product is formed and thus power generation performance of the fuel cells is deteriorated in power generation over a long period of time.
Therefore, the applicant proposes a fuel cell in which two intermediate layers formed of CeO2 solid solution having a rare-earth element excluding Ce is disposed between the solid electrolyte layer and the air electrode layer, in order to suppress the diffusion of Sr contained in the air electrode layer into the solid electrolyte layer or the diffusion of Zr contained in the solid electrolyte layer into the air electrode layer and to suppress the formation of a high-resistance reaction product (for example, see Patent Literatures 2 to 4).
In the course of manufacturing a fuel cell or generating electric power using the fuel cell in which two intermediate layers formed of CeO2 solid solution having a rare-earth element is disposed between the solid electrolyte layer and the air electrode layer, as described in Patent Literatures 2 to 4, when a large amount of rare-earth element excluding Ce in CeO2 solid solution contained in the intermediate layer is present in the solid electrolyte layer (particularly, around the interface of the solid electrolyte layer with the intermediate layer), ionic conductivity in a low temperature range (550° C. to 650° C.) around the interface of the solid electrolyte layer with the intermediate layer is particularly lowered, thereby causing a problem in that the power generation performance at a low temperature is lowered.    Patent Literature 1: Japanese Unexamined Patent Publication JP-A 2007-59377    Patent Literature 2: Japanese Unexamined Patent Publication JP-A 2008-78126    Patent Literature 3: Japanese Unexamined Patent Publication JP-A 2008-226653    Patent Literature 4: Japanese Unexamined Patent Publication JP-A 2008-226654